1. Field of the Invention
The present invention relates to an improvement of a mold clamping mechanism of an injection molding machine, and more particularly, to the construction of a stationary platen.
2. Description of the Related Art
As shown in FIG. 6, a mold clamping device of an injection molding machine is composed of a stationary platen 1′ set on a platform (not shown), a plurality of tie bars 2 fixed individually to the corners of the stationary platen, a movable platen 3 set for movement along the tie bars 2, and a mechanism for operating the movable platen 3. A stationary mold 4 and a movable mold 5 are set on the centers of the stationary platen 1′ and the movable platen 3, respectively. The stationary and movable molds 4 and 5 are opened and closed by a mold clamping device.
The stationary platen 1′ has its corners supported by the tie bars 2 and is configured to receive a reaction force in the center from the stationary mold 4. When a mold clamping force is applied to the stationary platen 1′, its mold mounting surface 6′ is deformed like a bowl, as indicated by two-dot chain line in FIG. 6, whereupon the contact surface pressure at the central parts of the molds drops inevitably. In some cases, therefore, burrs may be formed near the centers of the molds.
According to a technique described in Japanese Patent Application Laid-Open No. 2-214637, in order to solve this problem, a mold clamping force transmission portion 8 with a small outside diameter is disposed on the mold mounting surface of the stationary platen 1′, as shown in FIG. 7. A mold mounting plate 7 is attached to the stationary platen 1′ by means of the mold clamping force transmission portion 8, and a stationary mold is mounted on the mold mounting plate 7. In consequence, even if the stationary platen 1′ is deformed like a bowl by a mold clamping force, the deformation cannot be transmitted to the mold mounting plate 7 with ease.
According to a technique described in Japanese Utility Model Application Laid-Open No. 7-35020, moreover, a stationary platen of an injection molding machine is grooved so that its deformation cannot be transmitted to molds.
In a mold clamping device shown in FIG. 7, however, in fact, the pressure is concentrated on a region of the mold mounting plate 7 near the outer peripheral edge of the mold clamping force transmission portion 8 and drops gradually toward the center of the mold mounting plate 7, as shown in FIG. 8. Under the influence of this pressure concentration, the pressure at the central part of the parting surfaces 9 of molds tends to drop. In some cases, therefore, burrs may be formed near the centers of the molds, so that the mold mounting plate 7 must be thickened.
According to a technique described in Japanese Patent Application Laid-Open No. 2004-136506, furthermore, a mounting plate and a piston are fitted into a central opening (cylinder portion) of a stationary platen by insertion. Since a mold clamping force is transmitted from a compression chamber, however, then it propagates from the peripheral part of the mounting plate, so that the mold clamping force is reduced near a central bore of the piston. If the piston receives a reaction force when a nozzle is pressed against a mold with a shift cylinder for driving an injection unit connected with the aid of an adapter, the mold clamping force near the central bore is further reduced inevitably.